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Secondly, your puzzling bee is a very dark form of Bombus hypnorum, the Tree Bumblebee.

This species has only been in the UK since about 2001, but has been doing astonishingly well here.
It has already colonised England, many parts of Wales; and last year made it to Scotland.

It tends to use old bird nests in bird boxes as a nest location, but also uses house lofts - where it can use the loft insulation to keep the colony warm. So this is probably what your bees are doing behind the fascia boards.